Rotary helically splined actuators have been employed in the past to achieve the advantages of high-torque output from a simple linear piston-and-cylinder drive arrangement. The actuators typically employed a cylindrical body with an elongated rotary output shaft extending from end to end coaxially within the body, with an end portion of the shaft providing the drive output. Disposed between the body and the shaft is a piston sleeve splined to cooperate with corresponding splines on the body interior and the output shaft exterior. As an alternative to forming splines on the interior wall of the body, a separate ring gear attached to the body and encircling the sleeve may be used. The piston is reciprocally mounted within the body and has a head for the application of fluid pressure to one or the other opposing sides thereof to produce axial movement of the piston. The sleeve is elongated and coaxially receives the shaft therein.
As the piston linearly reciprocates in an axial direction within the body, the outer splines of the sleeve engage the splines of the body or ring gear to cause rotation of the sleeve. The resulting linear and rotational movement of the sleeve is transmitted through the inner splines of the sleeve to the splines of the shaft to cause the shaft to rotate. Bearings are typically supplied to rotatably support one or both ends of the shaft relative to the body.
With such an arrangement, as the piston reciprocally moves from one axial direction to the other to produce relative rotational movement between the body and the shaft in response to application of fluid pressure to the piston head, backlash results from the slack existing between the intermeshing splines of the piston sleeve and the body and the intermeshing splines of the piston sleeve and the shaft. While accurate machining of the splines will reduce the backlash problem, this procedure substantially increases the manufacturing cost. Even with accurate machining, conventional machining techniques are virtually incapable of totally eliminating the slack which produces the backlash problem. Furthermore, to the extent more accurate tolerances produce actuator parts which fit tightly together and reduce slack, assembly of the actuator becomes difficult. While accurate machining reduces slack initially, should the splined parts wear during usage or otherwise lose their original tolerances, no means exist for elimination of the slack that develops without disassembly of the actuator and possible remachining or replacement of the splined parts.
It is desirable that the fluid-powered rotary actuator should not require exceptionally accurate machining of the torque-transmitting parts to eliminate slack that produces backlash, and that the actuator should be easy to assemble. To facilitate assembly, the design of an actuator must incorporate a certain amount of slack, but the slack results in undesirable backlash during operation. Means should be provided for substantially complete elimination of the slack causing the backlash problem after the actuator is assembled. Even though accurate machining reduces slack initially, should the torque-transmitting parts wear during usage or otherwise lose their original tolerances, means should be provided for elimination of any slack that develops. Elimination of the slack should be accomplished in a simple manner without requiring disassembly of the torque-transmitting parts from the body, and with an adjustment which simultaneously removes the slack existing between all of the torque-transmitting parts within the body which translate linear movement of the piston into rotational movement of the output member. The actuator should be usable with torque-transmitting means other than splines to avoid the undesirably high frictional coefficient of splines.
While actuators have been constructed using balls and helical ball races, and provide a higher output efficiency due to the rolling friction of the balls being less than the sliding friction of the splines, conventional helical ball screw actuators require recirculation of the balls as the ball carrier reciprocally moves within the actuator cylinder. The recirculation allows the balls to roll relatively unrestricted within the ball races to avoid the balls scuffing along the races. While the use of recirculation eliminates most of the ball scuffing problem, it is difficult and expensive to manufacture an actuator with a recirculation path; and no recirculation path can provide a totally unrestricted flow of the balls. Additionally, to accommodate the recirculation path, the actuator must be made with a larger diameter than ordinarily necessary since recirculation requires that the recirculation path double back over the ball races carrying the balls.
When used in certain applications, such as for an aircraft flight control actuator, precision, weight and thickness becomes critical, while at the same time a high torque is required. When used as the power means to rotate an aircraft flight control surface, particularly with high performance aircraft, the forces encountered by the control surfaces are large, and accurate positioning and fast movement of the control surfaces is necessary due to the extreme responsiveness of the aircraft to control surface movement and to the high aircraft flight speeds. Also, since minimum turbulence is tolerable, it is desirable to use no external hinges or levers. Weight limitations are severe and space availability is limited. Every pound of weight has a negative impact on the flight performance of the aircraft, and little space is available to position actuators within the aircraft adjacent to the control surfaces. This is particularly a problem with current designs for thin wing aircraft where the interior space adjacent to the wing control surface, within which the actuator must fit, may be no larger than five inches in height and taper downward somewhat in the direction of the control surface. While a small diameter actuator is required to fit within the space available, high output torque and a large diameter shaft to handle the high torque are required.
If a conventional actuator is used having sufficient torque output and shaft size to handle the torque, it will be too thick to fit within the space available. If a small diameter conventional actuator is used, it will have insufficient torque or shaft size. Moreover, the weight of a conventional actuator would be too great. Another disadvantage of conventional actuators is the backlash realized. The backlash results in control surface flutter and inaccurate control of the control surface. Fluid leakage problems are also encountered with conventional actuators.
It is desirable to provide an actuator that avoids these problems, and that provides the benefits noted above. The present invention fulfills these needs and further provides other related advantage.